Laser diodes are solid-state emissive devices employed in a broad range of applications where highly coherent light is useful. While laser diodes offer a number of advantages for size, cost, and performance, however, these devices do not provide an output beam that is well-suited for use with systems that handle rotationally symmetric light, such as optical apparatus that use conventional spherical optics. The output beam of the laser diode, when considered in cross-section, has an aspect ratio that is highly asymmetric, with markedly different divergence angles in respectively orthogonal directions. The output beam emits from a wide stripe which extends along a “slow” axis to a width dimension that is several times its height along a “fast” axis that is orthogonal to the slow axis. Rotationally symmetric optics, meanwhile, are optimized for handling light beams that are themselves substantially symmetric. Thus, adapting the light from a laser diode or laser diode array to a spherical optical system can require components and techniques for rearranging the distribution of the light, such as by stacking beams of multiple lasers along the fast width, for example, to form a composite beam that has a more symmetric aspect ratio.
The innate asymmetry of the laser diode light is particularly disadvantageous for use with optical fibers. The highly symmetric input aperture of the optical fiber is poorly matched to the aspect ratio of the laser diode output beam, making it difficult to design an efficient optical system that can use all of the light output. In effect, when using light from a single laser diode, the input aperture of the optical fiber is readily over-filled in one direction and under-filled in the orthogonal direction.
One result of this innate incompatibility between the laser diode and optical fibers is that it imposes constraints on laser diode design. Laser diodes that are used to provide pump excitation light for fiber lasers, for example, are constrained in emitting stripe width, with an emitter width that is nominally no more than about 100 μm. Emitters having a wider stripe width, such as diodes with an emitter stripe width of 120 μm or longer, have been described and would be more efficient and provide proportionately more light. However, due to output beam geometry, the emitted energy in the slow axis direction would well exceed the input NA of the optical fiber in one direction (slow axis), while still under-filling the aperture in the orthogonal (fast axis) direction.
Thus, it can be appreciated that there is a need for a solution that rearranges the light output of a solid-state laser diode so that it is more compatible with symmetric optics and is better suited for efficient use with optical fibers.